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Old 02-10-2013, 02:34 PM   #1
p951951951t
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Default Cutting cat out of perrin downpipe, thinking of replacing with flex joint

I'm going to be porting my wastegate. After I finish, I think I'm going to cut the cat out of my perrin downpipe. I have some 3" SS exhaust tubing laying around, but I was thinking of adding a flex joint to take some strain off the turbo. I already put one into the catback right where it meets the downpipe, but I figure another can't hurt.

Anyone done this?
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Old 02-10-2013, 03:15 PM   #2
h00z0r
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Honestly I would sell your Perrin dp, and buy a no name down pipe, mod the new one and pocket the extra, those Perrin pieces are worth good money
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Old 02-10-2013, 03:58 PM   #3
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A flex joint in the downpipe? How exactly is this going to take strain off the turbo?

If you're set on doing this, catless downpipes are a dime a dozen.
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Old 02-10-2013, 06:17 PM   #4
p951951951t
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Because right now, the whole exhaust is rigidly connected to the turbo and mounted to the trans as well if I recall (don't really feel like goigng to peek under there at the moment)... Theoretically, shock generated from the exhaust's own inertia as well as from movements in the trans can put physical strain on the turbo and any parts attached to it, such as up-pipes.

I have heard a lot of people mention that aftermarket up-pipes have a lot of leaks, I wouldn't be surprised if that had something to do with it.

Then again, I'm still pretty new to the STi, I could be completely wrong. That's why I added the first flex joint anyhow.

Ihoozer: like the perrin piece, it's well made. I'd just as soon modify what I've got.

I'm cutting up the perrin either way, just trying to decide whether to weld in pipe or to add the flex joint.
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Old 02-10-2013, 06:54 PM   #5
bmw325ix
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I am very in favor of a flex section in the DP, the only probably is a good quality 3" flex section can get pricey.
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Old 02-10-2013, 07:53 PM   #6
p951951951t
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I think last time I paid like 40-50 for a SS flex pipe at summit racing.

The thing is I still have a 5' section of 3" SS tubing from summit I've never used, so if I decide on the flex joint I can just exchange that and only pay a few bucks out of pocket.
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Old 02-11-2013, 01:47 PM   #7
DeleriousZ
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Definitely agree with putting a flex pipe on the downpipe. Will save your up pipe, manifolds and everything else related to the exhaust.
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Old 02-11-2013, 09:35 PM   #8
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The cat in a Perrin down pipe is sized for a 6 liter engine. Not sure what you'd gain by removing it. It's made by FlowMaster. Take the number that's laser engraved on the cat and do a search on the FlowMaster site for more info.

Also aftermarket flex sections aren't noted for durability. If a flex was needed, why didn't the factory put one in? The stock downpipe is also rigidly mounted to the trans.
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Old 02-12-2013, 04:26 PM   #9
p951951951t
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I hear what you are saying, but the difference is enough of a restriction to prevent boost creep with all else being the same, which means it's having a tangible effect on the flow characteristics.

As for the factory... well, you have a point. Then again, how many people have experienced leaks when moving to an aftermarket up-pipe? Personally I think that's in part due to the rigid mount. I could be wrong, and I guess there is no way for me to know for sure. I've also seen several up-pipes with built in flex joints over the last few days - that may be the way to go.

Factory engineers make decisions for all kids of reasons... cost/simplicity/ease of manufacturing/etc. I do tend to be the type that thinks generally the engineers at the factory know what's best, but obviously sometimes they have to make compromises that an individual like myself might not.

What gives you concerns about the durability of a quality aftermarket flex joint? I've never had any trouble before but I'm interested in what you know.
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Old 02-12-2013, 06:33 PM   #10
DeleriousZ
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For anyone that's going to do a flex joint should make sure they use the right style as well.

This is not what you want:



This is what you DO want.



Notice how the 2nd one has the internal flex pieces? That style holds up to abuse much better than the 'regular' style of flex pipes.
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Old 02-12-2013, 09:14 PM   #11
RockNRace03
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im not so sure i agree with the flex joint. i can imagine it causing you more harm than good. they break all the time. even the "good" ones break and leak. i work for a custom exhaust shop and i would say 80% of our warranty repairs are from flex joints. we haven't used them in any products for the past year or so now. if you're looking for some extra flex, used a slip joint with accu-seal clamp
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Old 02-13-2013, 01:32 AM   #12
CalvinT
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Grimmspeed warns not to wrap a flex section, even theirs. Jeff Perrin is the one that warned me off after market flex sections. If you'll notice, they don't use them. The factory flex sections are more robust, have double walls.
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Old 02-13-2013, 09:04 AM   #13
p951951951t
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Yeah, the ones I use are the ones like the second pic, high quality stainless pieces.

Since I already have one just behind the downpipe, I'll probably just gut the cat or weld in some 3" SS pipe, especially now that I've seen the up-pipes with the flex joint are available and seem to work well.
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